Various types of biometric systems are used more and more in order to provide an increased security for accessing an electronic device and at the same time keep the user convenience at an acceptable level. In particular fingerprint sensors have been successfully integrated in such devices, for example, thanks to their small form factor, high performance and user acceptance. Among the various available fingerprint sensing principles (such as capacitive, optical, thermal etc.), capacitive sensing is most commonly used, in particular in applications where size and power consumption are important.
All capacitive fingerprint sensors provide an indicative measure of the capacitance between several sensing elements and a finger placed on the surface of the fingerprint sensor. Acquisition of a fingerprint image is typically performed using a fingerprint sensor comprising a plurality of sensing elements arranged in a two-dimensional manner, and a block based technique may be applied to the fingerprint sensor for acquiring a fingerprint image, where the blocks of sensing elements are sampled sequentially.
One of the problems associated with fingerprint sensors concerns so-called spoof fingers trying to mimic a live fingerprint to thereby deceive a fingerprint sensor. If fraud by the spoof finger is successful, unauthorized access to systems may undesirably be approved or unauthorized transactions may be approved which may lead to disastrous consequences. Furthermore, a spoof finger is relatively easy to produce which may eventually lead to an increasing number of fraud attempts, in particular as fingerprint sensors become more and more common as a means for authentication.
US2014270416 discloses a fingerprint sensing device with spoof detection capabilities which are interleaved with the acquisition of finger biometric data using sub-arrays of finger sensing pixels. A finger drive electrode is arranged adjacent to the finger sensing pixels to supply a voltage pulse to a finger or object (e.g. spoof finger) placed on the finger sensing pixels. For spoof detection, the finger or object is subjected to a voltage pulse via the finger drive electrode and the sub-array of finger sensing pixels is configured to sense the response from the finger or object. Spoof detection is thereafter based on the characteristics of the response which is affected by the conductance of the finger or object.
However, the spoof detection capability disclosed in US2014270416 is sensitive to the resistance of the finger (or the spoof object). The sensitivity to resistance of the finger also applies to the driving of the finger potential via the finger drive electrode for acquisition of finger biometric data, thereby reducing the quality of the acquired images. One way to reduce the influence of the resistance of the finger is to ground the finger and drive the circuitry associated with the drive signal. This way, images may be acquired which are less affected by the resistivity of the finger. On the other hand, this opens up for a larger selection of viable spoof materials due to the reduced sensitivity to the resistivity of the object or finger.
Thus, there is a need for improvement with regards to increasing the level of security in user authentication with fingerprint sensors.